1. Field of the Invention
This invention relates to methods for applying slurry or the like to a substrate in a discrete pattern, and particularly to a method for application of a phosphorescent slurry to a glass substrate for use as a color television raster.
2. Description of the Prior Art
The production of a color raster for a color television picture tube requires the application of primary phosphors or fluorescent substances such as red, green and blue to a substrate which is the inside portion of the screen area of a color picture tube. The method presently practiced in the art requires a sequence of repeated steps associated with the application of each phosphor, which is time-consuming and does not allow the reclamation of unused phosphor slurry for subsequent reuse in the method.
The currently practiced method involves coating the total surface of the substrate with a slurry of a first phosphor to be applied. The phosphor layer contains photosensitive material, such as photo-resist. Light is shown through a mask and the photosensitive phosphor layer is removed where not struck by the light, leaving a pattern of dots or stripes of the first phosphor to be applied. This process is repeated two more times in the case of color picture tubes in order to deposit a raster of primary color phosphors such as red, green and blue on the glass substrate.
The above method is known from German Pat. No. 1,447,791 and from German Auslegeschrift No. 1,462,653.
The above method possesses several disadvantages. Because of the repeated superimposed coating of the different phosphorescent slurries on the substrate, purity of the individual of the discrete slurry deposits on the substrate is diluted, and mixing of the slurries occurs to some degree, thereby resulting in inaccurate reproduction during use. Color impurities may be formed on the substrate glass directly, if residues remain after the rinsing of the non-exposed coated areas, as the rinsing pressure must not be too great so that the remaining fluorescent slurry deposits are not removed also.